The typical screw type slotted head fastener in use is driven by introducing a torque to a slot in the head of the screw by use of a screw driver. The same method is used whether the screw is driven clockwise or counterclockwise, whether tightening or loosening. The most troublesome faults with this method of driving screws are that the screw driver tends to come out of the slot under high torque and that the outer edges of the slot tend to chip or deform when slippage occurs or when repetitive installation and removal are necessary.
The typical screw driver has a tapered blade which fits into the screw slot, which, conversely, has parallel faces. The result of this combination is that the end of the blade is narrow by comparison with the width of the slot, allowing the screw driver axis to be at an angle with the axis of the screw. In addition, the contact between the screw driver and the screw is made at the top edge of the slot, where the screw contacts the tapered edge of the screw driver blade at two points. When torque is applied to the screw driver, this axial misalignment and point contact cause one component of the force applied to push the screw driver out of the slot. This is the first fault alluded to above. The second fault is related. When the screw driver slips out of the slot because of this mismatch of tapered and parallel surfaces, the top edge of the slot frequently chips or burrs because the point loading exceeds the material strength just prior to the blade clearing the slot. Further, even if wholesale failure of the material at the top edge of the slot does not occur, repeated application of point loading at the top of the slot frequently causes abrasion or creep of the material, resulting in a slot with rounded edges which is no longer capable of transmitting torque to the screw.
The prior art exhibits several attempts to alleviate these problems. The most relevant of these attempts are described below, but none are as effective at solving the problems as the present invention.
U.S. Pat. No. 3,923,088 exhibits a blade 20 with opposing concave surfaces 22 the purpose of which is to allow the lower edge 30 of the blade to bite into the screw slot faces to resist the tendency of the blade to leave the slot. This configuration removes material from the blade, causing it to be weaker than the present invention. Because of the complicated shape, manufacture of the screw driver would also be much more difficult. If failure of the blade occurs, the average user would not be able to restore it to its original shape.
U.S. Pat. No. 3,897,812 exhibits a similar configuration with complicated contours which suffers from the same drawbacks.
U.S. Pat. No. 3,405,748 shows a straight bit 5 with parallel surfaces. Its torsion tube construction requires that for a given bit width, the shank must be considerably smaller and weaker than that of the present invention, given the same material of manufacture. It also will require a greater number of more difficult manufacturing operations, and the tube would be far more difficult to clean, a feature which is anathema to the precision work in which such tools are frequently used.
U.S. Pat. No. 1,479,506 shows a blade with concave surfaces like those previously discussed, with the same drawbacks. U.S. Pat. Nos. 4,105,056 and 4,311,071 exhibit blades with thin sections in the center which will suffer from weakness compared to the present invention and which will be more difficult to manufacture and impossible to repair.
U.S. Design Pat. Nos. 112,592 and 229,475 show apparently flat bits which are also difficult to manufacture and impossible to repair.